The inlet opening ensures that gas can be supplied to the rotor chambers, more specifically into the spaces between the lobes of the helical rotors, whereby due to the rotation of these helical rotors the gas in these spaces can be compressed.
Two types of inlet openings are known, i.e. a radial inlet opening and an axial inlet opening.
An axial inlet opening is situated at the level of the end face on the inlet side of the housing.
Such an axial inlet opening ensures a supply of gas to the rotor chambers in the axial direction, i.e. along a direction parallel or essentially parallel to the axes of the helical rotors.
An axial inlet opening is situated very close to the vicinity of the (shaft) seals and bearings. This has the disadvantage that this typically leads to greater complexity and a requirement for longer rotor shafts.
A radial inlet opening is situated at the location of the cylindrical walls of the rotor chambers and ensures a supply of gas to the rotor chambers in the radial direction, i.e. along a direction perpendicular or essentially perpendicular to the axes of the helical rotors.
A radial inlet opening has the advantage that it is not only easy to realise, but also that the helical rotors are accessible via the opening for inspection, maintenance or synchronisation of the helical rotors.
It is known that the shape of the inlet opening must satisfy many requirements.
In order to be able to fill the spaces between the lobes of the helical rotors with gas to be compressed as optimally as possible, the inlet opening is preferably kept as large as possible, whereby it is ensured that the closure of the inlet opening happens at the right time due to the rotation of the helical rotors.
An idealised shape follows from these requirements, i.e. the ‘delta shape’, whereby a triangular inlet opening is defined from the shape of the lobes of the helical rotors so to speak.
This idealised shape has a few disadvantages.
Firstly such a large opening in the housing negatively affects the mechanical strength of the housing.
Secondly a connection must be made to an inlet pipe for the supply of the gas to be compressed, whereby the transition from the triangular inlet opening to the inlet pipe is technically very difficult to realise and whereby an inlet pipe with a very large diameter will be necessary.
It is clear that in practice such a ‘delta shape’ is rarely used for screw compressors. This idealised shape is often departed from by truncating the two base angles of the ‘delta shape’ such as described in NL 6.708.715, for example.
In other words the inlet opening is smaller, such that the mechanical strength of the housing is not jeopardized too much, while a good filling of the spaces between the lobes of the helical rotors is always obtained.
However, due to the rotation of the helical rotors during the operation of the screw compressor, turbulence occurs in the gas that is in the inlet opening or inlet zone, such that ‘mixing losses’ occur. Such losses are greater at high speeds of the helical rotors.
Due to these losses a proportion of the gas between the lobes of the helical rotors is thrown or blown away as it were. In other words: a proportion of the gas between the lobes will be lost, such that the efficiency of the screw compressor will decrease.
In order to prevent this a number of solutions are known, such as the use of ribs or partitions in the inlet opening to guide the flow of the gas to be compressed, as described in the utility model DE 7.611.162.
This has the disadvantage that a lot of flow resistance is generated without counteracting all mixing losses. Despite the guiding of the flow the spaces between the lobes cannot be filled optimally.
In DE 44.26.761 use is made of an axial inlet opening, whereby a delta-shaped recess or hollowing is made in the housing for the flow of the gas supplied so that an additional radial filling of the spaces between the lobes is obtained.
In order to prevent the turbulence or vortices in the delta-shaped recess, partitions or blades are affixed in the aforementioned recess.
Such a construction, not only with an axial inlet opening but also with the aforementioned partitions in the housing, is technically very difficult to realise.
In U.S. Pat. No. 4,488,858 use is made of a radial inlet opening in a truncated ‘delta shape’, such as in NL 6.708.715, whereby the truncated angles of the delta in the housing are milled or hollowed out but whereby a thin strip or edge between the truncated delta and both truncated angles is preserved.
As a result the traditional ‘delta shape’ of the inlet opening is created so to speak, whereby the two edges must limit the vortices.
However, these edges will prevent a good filling of the spaces between the lobes, as they ensure an at least partial or temporary closure of the hollowed parts of the housing when the helical rotors are rotating.
Moreover, mixing losses will still occur in the hollowed parts.